1. Field of the Invention
The present invention relates to a channel switching method and a CDMA communication system of a mobile communication terminal and a base station, using the same.
2. Description of the Related Art
In a mobile communication terminal of a CDMA (Code Division Multiple Access) type, there is a case that a physical data format is switched because of addition or removal of a service bearer, and change of a data communication rate or a spreading code or the like during a communication. Hereinafter, the switching is referred to as a channel switching. In such a case, the channel switching needs to be carried out at the same time by a network side such as a radio base station and a base station control station, and the mobile communication terminal.
FIGS. 1A and 1B show physical reception data formats before and after a traffic data communication rate is switched from 32 kbps to 64 kbps in a mobile communication terminal of a W-CDMA (Wideband Code Division Multiple Access) type, respectively.
Referring to FIGS. 1A and 1B, it is supposed that a data format before a rate switching is CH-A (a traffic data rate: 32 kbps, and a physical bit rate including a control signal: 120 kbps), and a data format after the rate switching is CH-B (a traffic data rate: 64 kbps, and a physical bit rate including a control signal: 240 kbps). Here, data is a traffic data, and TFCI (Transport Format Combination Indicator) and TPC (Transmit Power Control) are layer 1 control signals, and Pilot is a control signal used for determine establishment of synchronization with a base station (a known bit pattern: a pilot signal).
FIG. 2 shows a conventional channel switching sequence in the above-mentioned CDMA communication system. In FIG. 2, when a data format is switched from a data format (CH-A) to a different data format (CH-B) during communication using the data format (CHA) between the network side and the mobile communication terminal (MS: Mobile Station) (A1 of FIG. 2), a switching instruction from the data format (CHA) to the data format (CH-B) is firstly outputted from the network side to the mobile communication terminal (A2 of FIG. 2). At this time, a switching timing data is simultaneously outputted such that the switching operation is carried out at the same time in the network side and the mobile communication terminal.
The mobile communication terminal receives the switching instruction, and responds an acknowledgement (ACK) (A3 of FIG. 2). Then, the mobile communication terminal waits for the switching timing, and then carries out the switching operation from the data format (CH-A) to the data format (CH-B) (A5 of FIG. 2).
On the other hand, the network side confirms that the mobile communication terminal carries out the switching operation from the data format (CH-A) to the data format (CH-B), from reception of the acknowledgment (ACK). Then, the network side waits for the switching timing, and carries out the switching operation from the data format (CH-A) to the data format (CH-B) (A4 of FIG. 2).
After the switching to the data format (CH-B), the mobile communication terminal carries out a synchronization establishment determination of a reception signal, using the pilot signal of the data format (CH-B) and then notifies to the network side, the completion of the switching from the data format (CH-A) to the data format (CH-B) (A7, A8 of FIG. 2) at a time when the synchronization establishment is OK (normal) (A6 of FIG. 2).
In the above-mentioned conventional channel switching method, there may be a case of a failure of the channel switching operation, as shown in FIG. 3. At first, the network outputs the switching instruction to the mobile communication terminal to carry out the switching operation from the data format (CH-A) to the data format (CH-B) (B1, B2 of FIG. 3). At this time, the switching timing data is simultaneously outputted such that the switching operation is carried out at the same time in the network side and the mobile communication terminal.
The mobile communication terminal receives the switching instruction, and returns the acknowledgment (ACK) (B3 of FIG. 6). Then, the mobile communication terminal waits for the switching timing, and carries out the switching operation from the data format (CH-A) to the data format (CH-B) (B4 of FIG. 3).
Here, if the network side cannot receive the acknowledgment (ACK) from the mobile communication terminal for any reason, it is impossible to confirm that the mobile communication terminal carries out the switching operation from the data format (CH-A) to the data format (CH-B). Therefore, the network side does not carry out the switching operation from the data format (CH-A) to the data format (CH-B) even at the time of the arrival of the switching timing. In the mobile communication, there is a possibility that failure of signal reception frequently occurs because of fading, interference and the like in a radio interval.
On the other hand, the mobile communication terminal carries out the switching operation to the data format (CH-B), and then carries out the synchronization establishment determination of the reception signal using a pilot signal of the data format (CH-B). However, since the network side holds the data format (CH-A), the synchronization establishment is NG (abnormal) (B5 of FIG. 3).
After that, the network side uses a timer to output the switching instruction to the data format (CH-B) to the mobile communication terminal again (B6, B7 of FIG. 3). However, since having already switched to the data format (CH-B), the mobile communication terminal cannot receive the switching instruction. After all, the communication is disconnected (B8 of FIG. 3).
In conjunction with the above description, a spectrum spreading communications system is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-4211). In this conventional example, a carrier frequency is switched from a first frequency to a second frequency during communication with a first base station. After hand-over is carried out from the first base station to a second base station, a carrier frequency in communication with the second base station is switched from the second frequency to a third frequency. The second frequency is a frequency in the hand-over exclusive use.
Also, a CDMA mobile phone is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-69449). In the CDMA mobile phone of this conventional example, a transmission and reception section sends and receives a radio signal with a base station. A rake reception section receives a plurality of reception paths at a same time. A modulation and demodulation modulates a baseband signal to the radio signal and demodulates the radio signal in the baseband signal. A coding and decoding section encodes a communication signal and a control signal into the baseband signal and decodes the baseband signal into the communication signal and the control signal. A transmission unit and a sound coding section coverts a speech signal into the communication signal. A reception unit and a sound decoding section convert the communication signal into a speech signal. A control section controls an operation based on the control signal and sends necessary data to the base station as the control signal. The transmission and reception section receives signals sent out from the base station or a plurality of different base stations in two or more frequencies at a time, and transmits a signal to it or them.
Also, a channel estimating apparatus is disclosed in Japanese Laid Open Patent Application (JP-P2000-4212). In this conventional example, the channel estimating apparatus is composed of a first receiving section which receives a pilot signal on a first channel and a second receiving section which receives pilot signals on a second channel which are multiple-transmitted from a single antenna. The state of the channel is estimated from both of the received pilot signals.
Also, a synchronizing circuit is disclosed in Japanese Laid Open Patent Application (JP-P2000-244387). In the synchronizing circuit of this conventional example, diversity reception at a plurality of branches is carried out with a base station on communication. A correlation calculation is carried out to obtain correlation output for every branch, and the correlation outputs are combined. A signal from a base station as a switching destination is temporally stored, and the correlation calculation of the stored signal is carried out during a time period which the correlation calculation using any of the branches is not carried out, to establish synchronization with the switching destination.
Also, a base station is disclosed in Japanese Laid Open Patent Application (JP-P2001-8262). In this conventional example, a communication region covered by the base station is variable. The change of a spreading code is notified from a transmitting side to a receiving side between the base station and a mobile terminal using a channel, and after the notification, the spreading code is changed such that interference between the channel and another channel is reduced.